This invention relates to methods for tertiary recovery of petroleum from subterranean reservoirs, and more particularly relates to new aqueous alkaline flooding methods for displacing and recovering petroleum from a reservoir containing acidic crude oils.
Oil displacement from acidic oil reservoirs may conventionally be carried out by secondary or tertiary flooding processes such as aqueous alkaline flood processes in which an alkaline solution is injected into the oil reservoir to form surfactants by reaction with naturally occurring acids, inactivate polyvalent cations and to displace the oil from the sandstone and other constituents of the reservoir.
Substantial technical effort has been directed to the development of alkaline flood processes for tertiary oil recovery methods. Such efforts have been particularly directed to methods involving injecting an aqueous alkaline solution containing a surfactant, into the subterranean reservoir by means of injection wells, and withdrawing an oil-water mixture through one or more production wells. The patent literature describes injecting an aqueous alkaline solution to satisfy the surfactant adsorption sites on the reservoir rock and then injecting a surfactant-containing aqueous liquid which may also contain alkali. As discussed in U.S. Pat. No. 3,927,716, the alkali content may be adjusted to achieve the lowest interfacial tension between the flooding solution and the oil, which may occur at low alkali concentration. U.S. Pat. Nos. 3,804,170; 3,804,171 and 3,847,823 describe injecting aqueous alkaline solutions containing petroleum sulfonate surfactants which are formed by over-neutralizing petroleum hydrocarbon sulfonates. U.S. Pat. Nos. 3,977,470 and 4,004,638 similarly relate injecting aqueous alkaline solutions, which may be followed by an aqueous alkaline solution containing a hydrocarbon sulfonate surfactant as well as polyphosphates and carbonates.
U.S. Pat. No. 4,099,569 describes a staged process for recovering oil from a subterranean reservoir by injecting a surfactant solution in which the concentration of the surfactant is increased as increasing amounts of the solution are injected and then injecting a drive fluid. U.S. Pat. No. 4,232,737 describes a staged injection of a highly saline aqueous petroleum sulfonate surfactant system containing a solubilizing amount of cosurfactant and decreasing the concentration of both the salt and surfactant in stages to provide a trailing-edge salinity which is suitable for polymer thickened aqueous drive fluid.
However, such conventional methods of tertiary oil recovery have a variety of deficiencies in terms of cost, efficiency or deleterious side effects. For example, as suggested in U.S. Pat. 4,502,541, significant portions of alkali are consumed in conventional alkaline flooding processes by reaction with siliceous and carbonate minerals and consequent formation and dissolution of alkali metal silicates and carbonates. Such reactions are promoted at the condition of elevated temperature and pressure which may typically be present in the subterranean oil reservoir. Such reactions may cause the alkali to propagate slowly through the reservoir rocks, unnecessarily consuming the flooding solution components, and providing a solute component which is capable of precipitation and causing scale formation.
Such conventional alkaline flooding methods accordingly may encounter numerous problems which may diminish their oil recovery, cost effectiveness, and/or efficiency. For example, the precipitation of silicates and/or carbonates in the production well bore may be a significant disadvantage in such tertiary oil production methods. Such silicates and carbonates, which are produced and dissolved at the high temperature and pressure in the reservoir system may be readily precipitated out of solution in the production well as the pressure and temperature decrease upon withdrawal of the oil-flood water mixture from the production well bore. In this regard, as the petroleum/alkaline mixture rises to the surface, the pressure and temperature are dramatically decreased, gases are liberated, and the carbonates and/or silicates may precipitate to diminish or plug the bore.
As a result of cost and other deficiencies, tertiary recovery methods are not presently widely used in commercial production. Accordingly, there is a need for economical and effective aqueous flooding methods for tertiary oil recovery which would increase the recovery of petroleum from existing wells and otherwise nonproductive reservoirs. Accordingly, it is an object of the present invention to provide aqueous flooding methods for tertiary oil recovery which have improved economy, which reduce the detrimental effects of silicate and carbonate precipitation, and which are capable of efficient, effective and economical recovery of petroleum. These and other objects will be apparent from the following detailed description and the accompanying drawings.